I must admit that at first I was very confused by your question: the AmEx Rewards website doesn’t say anywhere that you can transfer your AmEx Rewards points to MTA travel. But I did a little digging and discovered that MTA Travel is a subsidiary of Helloworld Travel, which you can transfer your points to. So that is indeed an option.
… but in my opinion it’s a bad option. If you transfer your points to Helloworld, each point will be worth 1 cent or less (depending on which category of AmEx points you have). You’ll get much better value-for-points by transferring them to a Frequent Flyer programme and using them to redeem flights in premium cabins.
The very best option is to work out where you want to fly to, then do some research about how many points you’ll need in each programme to get there (you should check availability also), and transfer your points accordingly. However, if you don’t yet know where you want to fly to, Virgin (Velocity) and Singapore Air (KrisFlyer) are both very good options. I would lean towards Velocity if you’re mainly going to be travelling domestically, and KrisFlyer if you’re mainly going to be travelling internationally. Just be aware that KrisFlyer points expire 3 years after you transfer them, so if you don’t think you’ll be using some or all of them within the next 3 years, Velocity is the safer option.
Hope this helps!